1. Field of the Invention
The present invention relates to electronic cameras employing solid state pixel sensor arrays. More particularly, the present invention relates to circuits and techniques for exposure control of such cameras and to exposure control of such cameras by means of overflow detection in arrays of active pixels.
2. The Prior Art
Prior art exposure control techniques known to the inventors that use the actual image sensors during the actual exposure interval are of two types. Some prior art techniques integrate the total photocurrent by a common back-side electrode (anode) of a group of photodiodesxe2x80x94i.e., they integrate the substrate current to get an average light reading on the whole array. Other prior art techniques use nondestructive readout to sample selected pixels during the exposure interval, looking for an indication that some pixels are reaching a full-scale exposure.
The first technique is tricky and difficult to implement, since the photocurrents are small and the substrate is large and noisy. In addition, it responds strictly to the average light level across the image plane rather than to those pixels that are reaching a full-scale charge accumulation. The second technique requires a sequential polling, so is limited to either a very slow operation or to sensing only a very small subset of the pixels. The second technique is therefore not good for detecting the exact time when a small percentage of pixels are reaching a full-scale exposure.
Other prior art techniques for exposure control typically measure the light either at a different time, e.g. just before the actual exposure, or with a different sensor device that needs to be calibrated relative to the sensor that is picking up the actual image. Such techniques typically sample the image plane at selected fixed points rather than adapting to the lighting conditions of the entire image.
One such prior art technique uses an imager first to estimate a light level and thereby to calculate an optimum exposure duration for a second cycle of the imager. This technique is obviously not as fast, and particularly is unsuited to controlling the exposure time rapidly during a dynamic lighting event, provided for example from a strobe flash.
Another such prior art technique employs a separate overall light sensor to measure an average light level and to react to a sufficient quantity of light by closing a shutter or quenching a strobe flash. Mechanical shutters and non-frame-storage electronic sensors cannot be shuttered rapidly enough to use this technique during a flash, which is why the detector is sometimes used to turn off the light source instead of closing a shutter. These techniques require an awkward coordination between the camera, the light sensor, and the light source, and do not necessarily track automatically the sensitivity (or film speed) and lens aperture of the camera.
Another type of prior art technique relates to use of an adjustable overflow drain for dynamic range enhancement. These techniques have not been integrated with the use of the overflow current for terminating the exposure time. Variations on this technique employ either a moving overflow barrier or a dual exposure interval to increase dynamic range.
It is therefore an object of the present invention to provide an exposure control technique for an electronic still camera employing a solid state imaging array which overcomes the shortcomings of the prior art.
It is another object of the present invention to provide an exposure control technique for an electronic still camera employing a solid state imaging array which exploits the overflow current produced by overexposed pixels in active pixel arrays.
Yet another object of the present invention is to provide an exposure control technique for an electronic still camera employing a solid state imaging array which may be employed in conjunction with dark-frame-subtraction noise reduction techniques.
Yet another object of the present invention is to provide an exposure control technique for an electronic still camera employing a solid state imaging array which provides enhanced dynamic range through overflow detection in the active pixels.
A method according to the present invention for controlling the exposure of an active pixel array for applications such as an electronic still camera includes the steps of: integrating photocurrent in each pixel during an integration time period; collecting overflow charge from all pixels in the array during the integration time period; developing an overflow signal as a function of the overflow charge; and terminating the integration time period when the overflow charge exceeds a threshold level selected to represent a desired reference exposure level.
According to a presently preferred embodiment of the invention, the step of collecting overflow charge from all pixels in the array comprises setting a charge potential barrier at the cathode of a photodiode in each pixel, the potential barrier being lower than the potential at which charge would overflow into the substrate, and the step of developing an overflow signal comprises generating a signal from charge overflowing the potential barrier.
Apparatus according to the present invention for controlling the exposure of an active pixel array includes means for integrating photocurrent in each pixel during a integration time period; means for collecting overflow charge from all pixels in the array during the integration time period; means for developing an overflow signal as a function of the overflow charge; and means for terminating the integration time period when the overflow signal exceeds a preset threshold level.
According to a presently preferred embodiment of the invention,the means for collecting overflow charge from all pixels in the array comprises means for setting a charge potential barrier at the cathode of a photodiode in each pixel, the potential barrier being lower than the potential at which charge would overflow into the substrate, and for collecting charge overflowing the potential barrier. As presently preferred, this function is performed by using the reset transistor in each pixel as a settable charge overflow barrier which is set to a level below (more positive than) the potential at the substrate. In addition, the means for developing an overflow signal comprises means for developing a voltage proportional to the excess charge accumulation at the cathode of the photodiode in all pixels. As presently preferred, this function is performed by developing a voltage proportional to the total charge allowed to flow into the Vref supply from all pixels in the array.
Another aspect of the present invention provides for producing a dark frame for the purpose of canceling out fixed pattern noise. Dark frame subtraction is employed to significantly reduce fixed pattern noise due to variations between pixels. Dark frame capture can easily be implemented electronically with a frame store imager simply by having a very short exposure time, preferably as controlled by the same timing and logic circuits that control automatic exposure. This eliminates the need for a mechanical shutter to perform the dark frame generation, which will save cost and complexity of the camera. A method is provided for obtaining calibration information for the individual pixels. Normally each pixel in an imager is reset to a reference level before an integration cycle begins. After the pixel is reset, photocurrent in the photodiode causes the voltage on its cathode and a storage capacitor to droop, corresponding to the signal. To generate a reference dark frame, the reset switch and the transfer switch connecting the photodiode to the capacitor are clocked in rapid succession so that there is no time for photocurrent to accumulate, generating a reference frame that can be subtracted from the image frame at a later time when both frames have been stored on the host system. The dark frame captures information about readout offset voltages of the individual pixels and an absolute zero-intensity reference per pixel. The dark frame may be captured before or after an actual exposure frame. Gray frames for calibration may similarly be captured by varying the Vref potential during reset.